If you go with ethernet, you’ll need a wire. They’re generally allowed to be up to 100 meters (328 feet) long, so length should not be an issue.
You might want to try simple things too. Like if your Roku (what model number??) only works with 2.4GHz ethernet, then your router might only be transmitting 5GHz ethernet. But this is probably easily changed at the router. Ie: turn on the 2.4GHz Wi-Fi, make sure all modes (b/g/n) are enabled.
Keep in mind that many of the older lower-end Rokus that can only do 2.4GHz Wi-Fi also have no provision for adding an ethernet adapter. So, check models carefully before you start ordering ethernet stuff.
Banned but back. Because why not?